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Transcript
EMT 112/4 ANALOGUE ELECTRONICS 1 Power Amplifiers Syllabus Power amplifier classification, class A, class B, class AB, amplifier distortion, class C and D, transistor power dissipation, thermal management. 1 POWER AMPLIFIERS Part III (cont’d) Power amplifier circuits – Class AB 2 POWER AMP Class-AB Qn & Qp are assumed matched transistors Small biasing voltage to eliminate dead band 3 POWER AMP Class-AB Various techniques are used in obtaining the bias voltage VBB in class AB power amplifier circuit. 4 POWER AMP Class-AB – Diode Biasing The base-emitter of a BJT is basically a p-n junction and hence exhibits similar characteristics as that of a diode – voltage across a diode in a forward mode is almost constant over a range of the diode current. VD ID + VD - ID 5 POWER AMP Class-AB – Diode Biasing The above diode characteristic is utilised in biasing of the push-pull power amplifier In the absence of input signal (vI = 0), most of IBias flows through D1 and D2 establishing a small bias voltage VBB for the base-emitter junctions of Qn and Qp. 6 POWER AMP Class-AB – Diode Biasing When vI is at its peak +ve, iBn may be large. IBias shall be sufficient to supply both iBn and the current through D1 and D2 in order to maintain the bias voltage VBB. 7 17/03/08 8 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 A diode biasing class AB power amplifier is to meet the following specifications; • RL = 8 ; • output power to PL = 5 W; • peak output voltage to be not more than 80% of VCC; • minimum value of ID to be no less than 5 mA 9 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 (cont’d) For both Qn and Qp; I SQ 1013 A; 75 For D1 and D2; I SD 3 10 14 A Determine; a) IBias and VCC; b) The quiescent collector current c) iCn and iCp when the output voltage is at its peak positive value 10 Class-AB – Diode Biasing POWER AMP EXAMPLE 3 – Solution a) Vorms PL RL 5 8 6.32 V Vopeak 2Vorms 8.94 V VCC Vo peak 0.8 11.8 V Select VCC = 12 V 11 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) At the +ve peak of the output voltage; iEnpeak iL max Vo peak RL 8.94 1.12 A 8 iBnpeak iEnpeak 1 1.12 14.7 mA 76 12 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) To maintain a minimum of 5 mA through the diodes; I Bias iBn I D 14.7 5 19.7 mA Select IBias = 20 mA 13 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) b) Under quiescent condition (vI = 0), ID = 20 mA (neglecting iBn); VBB ID 2VT ln I SD 20 103 2 0.026 ln 14 3 10 1.416 V 14 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) Assuming Qn and Qp are matched transistors; VBEn VBEp VBB 0.708 V 2 Hence; I CQ I SQeVBB / 2VT 1013 e0.708/ 0.026 I CQ 67 mA 15 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) c) At the peak +ve value of output voltage; iEnmax iL peak 1.12 mA iBnmax 14.7 mA I D I Bias iBnmax 20 14.7 5.3 mA 16 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) VBB 5.3 103 2 0.026 ln 14 3 10 1.347 V iCn max 1 iEnmax 75 1.12 1 75 1.105 A 17 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) iCn max VBEn VT ln I SQ 1.105 26 ln 13 10 781 mV VBEp VBB VBEn 1.347 0.781 0.566 V 18 POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) iCp I SQe VBEp / VT 1013 e0.566/ 0.026 0.285 mA Hence, when the output voltage is at its peak positive value; iCn 1.105 A and; iCp 0.285 mA 19 POWER AMP Class-AB – Diode Biasing EXERCISE 1 For the same amplifier as in Example 3 above, find iCn and iCp when the output voltage is; a) +5 V; b) -5 V; c) at its peak negative value. 20 POWER AMP Class-AB – VBE Multiplier Biasing The biasing circuit comprising Q1, R1, R2 and IBias, provides the biasing voltage VBB. Neglecting the base current of Q1; VBE1 IR R2 VBB I R R1 R2 R1 VBE1 1 R2 Thus, VBB can be set by selecting suitable values for R1 and R2. 21 POWER AMP Class-AB – VBE Multiplier Biasing A fraction of IBias flows through Q1, so that; I C1 VBE1 VT ln I S1 Under quiescent condition, iCn and iCp are small and hence iBn and iBp are negligible. As vI increases, iCn increases followed by an increase of iBn. IC1 decreases but VBE1 is almost constant. 22 POWER AMP Class-AB – VBE Multiplier Biasing To facilitate adjustment of the ratio R1/R2 and hence, the value of VBB, a third resistor Rv is included in the circuit. 23 POWER AMP Class-AB with Input Buffer Transistors R1, R2 and the emitter-followers Q1 and Q2 establish the required quiescent bias. R3 and R4 (usually of low values) are incorporated to provide thermal stability. The output voltage is approximately equal to the input voltage (emitter-follower) 24 POWER AMP Class-AB with Input Buffer Transistors When the input voltage vI increases, the base voltage of Q3 increases and the output voltage vO increases. The emitter current of Q3 increase to supply the load current iO. The base current of Q3 increases. The increase in base voltage of Q3 reduces the voltage across, and the current through R1. This means that iB1 and iE1 also decrease. 25 POWER AMP Class-AB with Input Buffer Transistors Also when the input voltage vI increases, the voltage across R2 increases and iE2 and iE2 increase. The input current iI accounts for the reduction in iB1 and the increase in iB2 i.e. iI iB 2 iB1 (Kirchhoff’s Current Law) 26 POWER AMP Class-AB with Input Buffer Transistors Neglecting vR 3 , vR 4 , iB 3 and iB 4 we have; vI VBE V iB 2 1 n R2 and; V vI VEB iB1 1 p R1 27 POWER AMP Class-AB with Input Buffer Transistors If; V V , VBE VEB , R1 R2 R and; n p then; vI VBE V V vI VEB iI 1 R 1 R 2v I 1 R 28 POWER AMP Class-AB with Input Buffer Transistors Since the voltage gain is approximately unity, the output current is; vO vI iO RL RL The current gain is; iO 1 R Ai iI 2 RL which is quite substantial. A large current gain is desirable since the output stage must meet the power requirements. 29 24/03/08 30 POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 (a) Determine the quiescent bias currents in all transistors; (b) Calculate all the currents labeled in the figure and the current gain when vI = 10 V. 31 POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (a) For vI = 0 (quiescent currents); iR1 iR 2 iE1 iE 2 15 0.6 7.2 mA 2 Assuming all transistors are matched, the bias currents in Q3 and Q4 are also approximately 7.2 mA since the base-emitter voltages of Q1 and Q3 are equal and those of Q2 and Q4 are equal. 32 POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) (b) For vI = 10; Because the voltage gain is approx. unity; vO vI iO RL RL 10 100 mA 100 iE 3 iO 100 mA iB 3 iE 3 100 1.64 mA 1 61 33 POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) V VBE vI iR1 R1 15 0.6 10 2.2 mA 2 iE1 iR1 iB3 2.2 1.64 0.56 mA iE 1 0.56 iB1 9.18 μA 1 61 34 POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) Since Q4 tends to turn off when vI increases, iB4 is negligible. Therefore; i E 2 iR 2 vI vEB V R2 10 0.6 15 2 12.2 mA iB 2 iE 2 12.2 mA 200 μA 1 61 35 POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) The input current; iI iB 2 iB1 200 9.18 191 μA The current gain; iO 100 Ai 524 iI 0.191 36 POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) If the previous expression i.e. Ai we have; Ai 1 R 2 RL is used, 1 60 2 610 2 0.1 The higher gain is due the fact that the base currents of Q3 and Q4 are neglected in deriving the expression. 37 POWER AMP Class-AB with Input Buffer Transistors EXERCISES Problems 8.36 through 8.38 in the text book 38